The invention relates to a knife shaft pair for cutting web-like material, in particular undulated board, wherein the knives contact each other temporarily under bias, if necessary, shortly before termination of the cutting operation.
In cross-cutters for undulated board manufacturing systems knife shaft pairs are normally used, wherein each shaft pair holds at least one knife with a uniformly extending cutting edge which has an equal spacing from the shaft axis. The knife blade extends either in parallel to the shaft axis or is arranged helically. The helical arrangement has the advantage that during the cutting operation always only a short section of the cutting edges is in engagement with one another at a single instant, whereby the loading on the knives, the knife retainer and the shafts is maintained low. The cutting edges of the knives describe circles which intersect slightly. The cutting edge of the one knife is in the leading area and the other in the trailing area of the knife blade. The radius of revolution of the cutting edge in the trailing area is slightly larger than the radius of the other cutting knife, so that only a single contact of the cutting edges occurs during one revolution and the cutting edges are moved apart relatively quickly after that contact. To ensure an unobjectionable cut, the cutting edges must be biased towards each other, i.e. they abut against one another under a partially considerable bias during the contact.
Such knife shaft pairs have generally proved satisfactory. With relatively tough material or from a certain grammage of the undulated board on or also with moist or coated undulated board a considerable bias is required to obtain an unobjectionable cut. In a boarderline case, no clear cut is obtained at all. A high bias necessitates high cutting forces. High cutting forces necessitate an adjustment of the knives at relatively short intervals, which causes an undesirous interruption of production. High cutting forces also cause rapid wear of the knives.
Hence, the object of the invention is to provide a knife shaft pair for cutting web-like material, in particular undulated board, which enables longer adjustment intervals and a longer service life.